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March 10, 1964- T. H. MOORE ELECTROSTATIC PRINTING Filed Dec. 2, 1959 2 Sheets-Sheet l fl NMEA- A INVENTOR. THEMAS H MDDRE March 10, 1964 T. H. MOORE ELECTROSTATIC PRINTING 2 srieets-sheet 2 Filed Deo. 2, 1959 INVENTOR.

THEMAS H MUURE iA/Ef United States Patent Office 3,124,456 Patented Mar. 10, 1964 3,124,456 ELECTRUSTATIC PRINTHJG Thomas Il. Moore, Franklin Park, NJ., assigner to Radio Corporation of America, a corporation of Delaware Filed Dec. 2, 1959, Ser. No. 856,841 5 Claims. (Cl. 96-1) This invention relates generally to electrostatic printing and particularly to improved recording elements and improved methods of electrostatic printing utilizing said improved recording elements. This application is a continuation-in-part of my copending application, Serial No. 770,631, led October 30, 1958.

An electrostatic printing process is that type of process for producing a visible recording, reproduction or copy which includes as an intermediate step, converting a light image or electrical signal into an electrostatic charge pattern. The process usually includes the conversion of the charge pattern into a visible image which may be a substantially faithful reproduction of an original, except that it may be of a different size, or contrast value.

A typical electrostatic printing process may include producing an overall electrostatic charge on the surface of a photoconductive material such as selenium, anthracene or zinc oxide dispersed in an insulating binder. A light image is projected on the charged surface, discharging the portions irradiated by the light rays while leaving the remainder of the surface in a charged condition to thus form an electrostatic image. The electrostatic image is rendered visible by applying a developer powder which is held electrostatically to the charged areas of the surface. The powder' image thus formed may be fixed directly to the photoconductive material or may be transferred to another surface upon which a reproduced image may be desired and then xed thereon. The xing step commonly comprises fusing the developer powder to the photoconductor material by the application thereto of heat. For a more detailed description of electrostatic printing, reference is made to Electrofax-Dlrect Electrophotographic Printing on Paper, by C. J. Young and I-I. G. Greig, RCA Review, vol. 15, No. 4.

It is an object of this invention to provide improved electrostatic printing processes and apparatus.

Another object of this invention is to provide an electrophotographic recording element of improved sensitivity.

A further object is to provide improved methods and means of electrostatic printing which do not require special precautions to prevent exposure of the recording element to ambient light.

Another object is to provide improved methods and means of printing utilizing a photosensitive element having a faster speed of response than those previously used in f conventional electrostatic printing processes.

Still another object is to provide an improved electrophotographic element having specific electrical properties which impart to said element an improved sensitivity and speed of response.

In general, the foregoing and other objects and advantages are accomplished in accordance with the invention by providing an improved recording element for electrostatic printing comprising a support member having on one surface thereof a layer of photoconductive material which is overcoated with a layer of insulating material. The layer of insulating material has a dielectric constant/ thickness ratio at least equal to and preferably substantially larger than that of the photoconductive layer. Also, the insulating layer is selected to have a Volume resistivity at least equal to that of photoconductive layer. Either the -support member or the insulating layer is substantially transparent to permit exposure of the photoconductive layer to incident radiation.

Because the functioning of the recording element of this invention is dependent on the dielectric constant/ thickness ratios and resistivities of the insulating and photoconducting layers, it is important that these layers be in intimate contact. Should any air or gas gap exist between any portions of the two layers the effective dielectric constant/ thickness ratios thereof would be upset and functioning of the recording element substantially impaired. Thus, in a practical sense, it is essential that the two layers must be formed as an integral unit. This can easily be accomplished by coating, spraying or evaporating insulating material onto the photoconductive layer to form the insulating layer.

Also included is a method of electrostatic printing employing the improved recording element of this invention. The method contemplates exposing the Photoconductive layer to a light image while simultaneously or subsequently applying an electric field across the photoconducting layer and the insulating layer. The eld is applied for a time about equal to the time constant of the RC network represented by an incremental area of the insulating layer in series with a corresponding incremental area of the photoconductive layer.

The invention and characteristic embodiments thereof will be described in greater detail by reference to the accompanying drawings wherein similar characters are applied to similar elements, and in which:

FIGURE 1 is a sectional view of a recording element for electrostatic printing constructed in accordance with the invention;

FIGURE 2 is a sectional View of a modification of the recording element of FIGURE 1;

FIGURE 3 is an electrical equivalent circuit represent` ing the recording element of FIGURE 1;

FIGURE 4 is another electrical equivalent circuit representing the recording element of FIGURE 1;

FIGURES 5 and 6 are graphs relating to the functioning of recording elements such as those of FIGURES l and 2;

FIGURE 7 is a sectional view of a camera that may be used with the recording element of FIGURE 1 to carry out the methods of the invention; and,

FIGURE 8 is a pictorial representation of a method for developing an electrostatic image formed on the recording element of FIGURE 2.

As shown in FIGURE l, a recording element 18 according to the invention comprises a relatively conductive support member 20 such as a metal plate, paper sheet, aluminized paper, etc. coated on one side with a photoconductive material 22, which in turn is overcoated with a thin transparent insulating dielectric material 24. The support member 20 has a volume restistivity less than that of the photoconductive material.

The photoconducting layer 22Ais placed contiguousrwith the support member 20. For purposes of illustration, the photoconducting layer 22 comprises a coating of substantially uniform thickness, of .0002 to .004 inch, upon the support member 20. The photoconducting layer 22 comprises, for example, photoconducting powdered cadmium sulfide, powdered cadmium selenide, or sintered cadmium selenide. A detailed description of a method of making a photoconducting layer of sintered cadmium selenide, for example, is disclosed by Thompsen in U.S. Patent 2,765,385. Photoconductive layers including zinc oxide are described in the Young and Greig publication,

op. cit.

at the junction between the photoconducting layer 22 and the insulating layer 24. rThis is about 2 microns or less. The insulating layer comprises, for example, Vinylite VYHH, applied by spraying. Vinylite JYHl-I is a polyvinyl chloride-acetate resin manufactured by the Bakelite Corporation, i ew York, NY., and its approximate composition is 87 percent polyvinyl chloride and 13 percent polyvinyl acetate. Other suitable insulators include polystyrene, Lucite, polyvinyl chloride and polyvinyl acetate.

ln FIGURE 2, another recording element 11d comprises a transparent electrically conductive backing Z such as an electrically conducting glass plate coated on one side with photoconductive material 22, which in turn is overcoated with a thin insulating dielectric material 24. The backing 20 may be a glass plate to which an electrically conductive coating 20'@ such as NESA coating, marketed by the Pittsburgh Plate Glass Co., Pittsburgh, Pennsylvania, is applied. This coating Zila may be produced by treating a glass sheet with tin chloride. Other transparent electrically conducting coatings, for example, thin iridescent metallic coatings may also be used.

Specific examples of improved electrophotographic recording elements include the following:

Example I (FIGURE I) A sheet of paper has a coating thereon comprising a photoconducting layer of Zinc oxide in a binder. This layer has a thickness of about 10-3 cm., a dielectric constant of about 5 and a volume resistivity of about 10l2 ohm-cm. in darkness. This layer is overcoated with one of the following materials to form an insulating layer having the approximate measured electrical properties indicated:

Dielectric Minimum Materials Goisgant Maximum Thickness (D) Resi(st)ivity er p Vinylite 2.7 to 3.56--- 5.5 4 to 7.1X10-4 em--- 1011 ohm-cm.

VYHH Polysty- 2.45 to 3.6-. 5.1X10-4 to 7.2X10-4 em-. 1014 ohm-cm.

rene. Lucite 2.57 to 4.5 5.1 104 to 9.0X10-4 cm 1015 ohm-em. Polyvinyl 3.0 to 6.4.--- 6.6Xl0-4 t0 1.3)(10-3 cm 1012 ohm-cm.

Chloride Polyvinyl 2.7 to 6.1-.-- 5.5 104 to 1.2)(10-3 cm 1012 ohm-cm.

.Acetate Example II (FIGURE I) A metal plate 20 has a photoconductive coating 22 thereon comprising sintered cadmium selenide. This coating has a thickness of about 10-3 cm., a dielectric constant of about 3 and a dark resistivity of about 109 ohm-cm. (resistivity may be varied from 1012 ohm-cm. to as low as 10 ohm-cm. by varying quantities of incorporated impurities such as, for example, chlorine and cop- Example III (FIGURE 2) A glass plate 20 has evaporated thereon a thin conductive layer 20%! of metal or tin chloride. This layer Zil'a is overcoatcd with powdered cadmium suliide which is bonded to the conductive layer Zila with a binder such as, for example, ethyl cellulose. The cadmium sulfide layer 22 has substantially the same electrical properties as the sintered layer 22 of Example H. Hence, the cadmium sulde layer may be provided with an insulating overcoat in the manner set forth in Example II.

ln all computations herein, area dimensions of the i photoconductive and insulating layers are ignored. Since the two layers are contiguous, area dimensions do not enter into any of the computations involving an elcmental area of the insulating layer overlying a corresponding area of the photoconductive layer,

FIGURE 3 illustrates in schematic form an electrical circuit which characterizes the recording element of FIGURE 1. This equivalent circuit shows the two elements: the insulator to provide for charge storage and the photoconductor for transforming changes in light intensity into changes in electrical resistance thereby controlling the quantity of charge applied to the insulator. In FIGURE 3, the electrically conductive layer or support member 20 is shown pictorially, since it is a good conduct-or and represents only a common connection. Any elemental segment of the photoconducting layer 22 which is coated upon the support member 20 may be represented by a parallel combination of a variable resistor 41% and a capacitor 46. The resistor 44 represents the electrical resistance of an elemental segment of the photoconducting layer, and its value depends upon the intensity of incident light upon the photoconducting layer 22. The capacitor 46 represents the capacitance of this same elemental segment of the photoconductive layer, and its value depends upon the photoconductor material and its dimensions.

Coated over the photoconducting layer 22 is the insulating layer 24 whose equivalent circuit for each elemental segment may also be represented by a parallel combination of a iixed resistor 4S and a capacitor 50. The resistor 48 represents the resistance of an elemental segment of the insulating layer, and the capacitor Sii represents the capacitance of this same elemental segment. This network is connected in series with the network representing the photoconductive layer, since both layers are physically superposed on one another in the recording element. The equivalent circuit of FIGURE 3 is a lumped constant approximation of a system which actually has distributed constants. The networks shown in dotted configuration indicate that a plurality of such networks are required to approximate the recording element over its entire area,

In accordance with the invention, by properly selecting the characteristics of the materials in the recording element, the equivalent circuit of FIGURE 4 is obtained. The desirability of this type of circuit will be explained hereinafter. The elemental capacitor 46 of the photoconductive layer and the element resistor 48 of the insulating layer have been neglected in FIGURE 4. The capacitor 46 can be neglected if it is made small compared to the capacitor 50, and the resistor 48 can be neglected if it is made large compared to the resistor 44. Since the resistor 48 represents the resistance of an insulator, which is inherently very large, it can be neglected when compared to the elemental resistance of the photoconductive layer.

The value of the capacitor 46 can be neglected if it is made substantially smaller than the value of the capacitor 50. This can be achieved by suitable selection of the photoconductor and insulating layers. For example, assume the photoconductor selected is cadmium sulfide and it is desired to minimize its equivalent elemental capacitance 46. The photoconductive layer should be made as thick as feasible since capacitance is inversely proportional to the thickness. The maximum useable thickness of the photoconductive layer is fixed by the depth of light penetration into the layer. For cadmium sulfide this limit is about microns. If, as in Example II, the cadmitun sulde coating has a dielectric constant of about 3, then the elemental capacitor 46 will have a capacitance of approximately 265 enfarads/cm?.

Since the value of the elemental insulator capacitor 50 is also inversely proportional to the thickness of the insulating layer, it is desirable to make this layer as thin as possible. By selecting an insulator having a thickness such as, for example, 2 microns and a dielectric constant of about 5, an insulator elemental capacitance of 2200 ,u.,u.farads/cm.2 is obtained. Thus, in this example, the insulator capacitor Sil is about 8 times larger than the photoconductor capacitor 46.

To illustrate in FIG. 4 how the improved recording element thereof having the equivalent circuit shown may be used for electrostatic printing, one terminal of a charging source illustrated as a battery 52 shown connected to the electrically conducting layer 2@ and the other terminal is connected throughA a switch 54 to the insulator layer. A lens 56 projects a radiant image such as a light image upon the recording element. Now, the amount of current that will flow through each series resistor-capacitor network when the switch 54 is closed depends upon the instantaneous magnitude of the photoconductor resistor 44, which in turn depends upon the amount of light incident on an elemental area of photoconductor. Coincident with or after the photoconductor is thus selectively exposed to light, a potential is applied across the recording element as by closing the switch 54. The switch is closed for a time less than that necessary to fully charge the insulator capacitor 50. This results in the photoconductor selectively modulating the amount of charge stored in the insulator capacitor in accordance with the amount of light incident upon each elemental photoconductor area. When the switch 54 is opened, the charge on the insulator capacitor 50 remains permanently stored since there is now no discharge path. Thus a latent electrostatic image is formed on the insulator layer.

The graphs of FIGURES 5 and 6 further illustrate the build-up of charge across the insulator 24 of FIG. 1. An initial measurement is taken at a time, t1, on initiation of charging. At time t1, the voltage VL across light exposed areas of the insulator 24 is substantially equal to the voltage VD across unexposed areas. Thereafter VL increases more rapidly than VD until a maximum difference (Vsigmx) between VL and VD is obtained at a time tom. If charging of the recording element is continued, VL will reach a. maximum which will then be rapidly attained by VD.

The graph of FIG. 5 illustrates charge build-up on the recording element of FIGURE l wherein the capacitance C, of the insulating layer 24 equals the capacitance Cp of the photoconductive layer 22. The graph of FIG- URE 6 illustrates charge build-up on the recording element when C1=2Cp. In both iigures, layers are chosen so that the resistance Rd of unexposed areas of the photoconductive layer 22 is ten times the resistance of RL of exposed areas.

From the graphs it is evident that maximum voltage differential between exposedA and unexposed areas will result when C, is as large as possible compared to Cp, i.e., when one provides a recording element which approaches as nearly as possible the ideal conditions illustrated in FIG. 4.

The graphs illustrate qualitatively that there is an optimum charging time (rapt) during which the maximum voltage difference across the insulator layer between exposed and unexposed areas is developed. This optimum charging time is directly dependent upon the time constant of an RC network representing the equivalent circuit through an incremental area of the recording element. This time constant equals RL(Cl-Cp). A

quantitative expression for optimum charging time takes the following form:

t stammen 1n R13/R..

ent- RD RL When RD is very large compared to RL and Ci very large compared to CD this expression can take the following simplied form:

toptIRLCi 111 R13/RL The latter expression is a suciently accurate means for determining charging time in accordance with this invention.

A camera and additional accessories for utilizing the novel recording element 18 of FIGURE 2 is shown in FIGURE 7. The camera comprises a lens 26, a bellows 2% and an enclosure 30 for the recording element 18. The recording element is placed in the enclosure 30 and held firmly in position by a pair of insulating spring members 34. The recording element 18 as described heretofore comprises the transparent electrically conducting layer 2li', the photoconducting layer 22 upon said transparent layer 20', and the thin dielectric insulating layer 24 upon the photoconductor layer 22.

Located in the enclosure 30 and directly behind the insulating layer 24 is a corona discharge electrode 36. This electrode may comprise a plurality of 3 mil corona discharge wires 38 spaced about 0.5 inch from each other and from the insulating layer 34. The wires 3S are housed in a metal shield 4@ in order to direct the corona discharge upon the insulating layer 24. The corona discharge wires 3S are connected to one terminal of a voltage source illustrated as a battery 42. Another terminal of the battery is connected through a switch 43 to the enclosure 3i?. By making the enclosure Si? of electrically conductive material, the layer 20 is connected through the switch 43 to the positive terminal of the battery 42. As an example, a voltage of the order of 6090 volts when applied to the wires 38 produces a corona discharge therefrom. The recording element i8 is placed in the camera so that the transparent layer 29 is closest to the lens 26 and a light image therefrom will be incident upon the photoconductor layer 22.

A similar exposure procedure to that described heretofore is utilized with the camera of FIGURE 7 to obtain a latent electrostatic image on the recording element. For example, the image 42 is tiret focused on the recording element 18. The switch 43 is then closed for a specied time during or after exposure. When the switch i3 is opened, an electrostatic image is recorded on the insulating layer by the mechanism heretofore discussed.

Unlike conventional silver halide photography, the recording element does not undergo an irreversible change upon exposure to light. A latent electrostatic image will be produced on the insulator layer only when a light image isl focused on the recording element and the corona discharge is applied thereto. Thus, it can be seen that special precautions against exposure to light are unnecessary during manufacture and distribution and between charging and development of the recording elements. With a Vinylite VYHH insulating layer 24', the exposed recording element could be allowed to remain in ambient light for several weeks before development.

Another important feature of this invention is that the recording element is exposed strictly on a transient basis, meaning that there is no direct current through the recording element. Consequently the dark current properties of the photoconductor layer are not critical since such properties affect exposure time only. As a result, photoconductive materials which have light sensitivities several orders `of magnitude better than other commonly used electrophotographitc materials may be used. For example, a cadmium sulde photolayer has a sensitivity approximately 800 times that of an electrophotographic material such 7 as photoconductive zinc oxide dispersed in an insulating binder.

The electrostatic irnage may be stored for a time if `desi-red. Ordinarily the next step is to develop the electrostatic image with a finely-divided developer substance such as a finely-divided powder or an ink mist. Referring to FIGURE 8, development of the electrostatic image is preferably accomplished by passing a developer brush 56 containing la developer powder across the surface of the insulating layer 24 bearing the latent electrostatic image. Developer powder 58 is deposited on those areas of the surface reta-ining an electrostatic charge. The developer brush comprises a mixture of magnetic carrier particles, for example, powdered iron and the developer powder.

A preferred carrier material for the developer mix consists of alcoholized iron, that is, iron particles free from grease and other impurities soluble in alcohol. These iron particles are preferably relatively small in size, being in their largest dimension about .O02 to .008. Satisfactory results are also obtained using a carrier consisting of iron particles of a somewhat Wider range of sizes up to about .OOil to .020".

A preferred developer powder may be prepared as follows. A mixture comprising 200 grams of 200 mesh Piccolastic resin 4358 (an elastic thermoplastic resin composed of polymers of styrene, substituted styrene and its homologs), marketed by the Pennsylvania Industrial Company, Clairton, Pa., 12 grams of Carbon Black G, marketed by the Eimer Iand Amend Co., New York, NY., 12 grams of spirit Nigrosine C.S.B., marketed by the Allied Chemical and Dye Co., New York, N.Y., and 8 grams of Iosol Black, marketed by the Allied Chemical and Dye Co., New York, N.Y., are thoroughly mixed in a stainless steel beaker at about 200 C. The mixing and heating should be done in as short `a time as possible. The melt is poured into a brass tray and allowed to cool and harden. The hardened mix is then broken up and ball milled for about 20 hours. The powder is screened through a 200 mesh screen and is then ready for use as a developer powder. This powder takes on a positive electrostatic charge when mixed with glass beads or iron powder. It therefore develops an electrostatic image composed of negative charges. A magnetic carrier to developer powder ratio of about 100 to 1 is preferred, although this ratio may vary as widely as between 250 to 1 and 25 to 1 depending on the particular components selected.

The developer powder may be chosen from a large class of materials. The developer powder is preferably electrically charged to aid in the development of the electrostatic latent image. The powder may be electrically charged because the powder (1) is electroscopic, or (2) has interacted with other particles with which it is triboelectrica'lly active, or (3) has been charged from an electric source such as a corona discharge. Examples of suitable developer powders are powdered zinc, powdered copper, carbon, sulphur, natural and synthetic resins or mixtures thereof.

The developer powder may be applied to the image in other ways, for example, it may be dusted onto the image, or it may be mixed with glass beads or other suitable carrier particles and then brought into contact with the surface of the printing base. The beads may serve as a temporary carrier, releasing the powder particles upon contact with the charged surface.

Still another method of applying developer powder to the image is by use of a magnetic brush as described in the publication, Electrofax-Direct Electrophotographic Printing on Paper, op. cit. In this method a developer brush is formed by dipping a bar magnet into a mass of developer mix comprising iron particles mixed with a ycarbon pigmented rosin powder in the proportion of about to 1 by weight. The bar magnet may be in any convenient form, .either electromagnetic or permanent.

When the magnet is withdrawn from the mass of developer mix, a quantity of the mix adheres by magnetic attraction to the magnet forming a brush like mass.

To develop the latent electrostatic image, the developer brush is lightly rubbed across the image surface, causing particles of the pigmented resin to transfer from the brush to the charge image, thereby producing a direct powder image.

The composition of a developer mix for use with the magnetic brush is not critical. While a preferred mixture comprises about 2 percent by weight developer powder, the remainder being iron, satisfactory results may be obtained using a mixture comprising 1 percent to 6 percent by weight developer powder.

The developed image 58 is now fixed to the insulating layer 24%. Ilf the developer powder has a relatively low melting point, the image may be xed by heating, for example, with an infra red lamp to fuse the powder to the surface. The powder image is preferably fused through the insulating layer 2d. Sulphur or synthetic resin powders may be fixed in this way. Alternatively, the powder image 58 may be pressed into the layer 24. Another method of fixing the powder image 58 is to -apply a thin coating of a solvent for the material of the powder image 58. The solvent may soften the developer powder particles and cause them to adhere to one another and to the layer 2.4i. Alternatively, a solvent may be used to soften the insulator layer 2li and cause the developer powder particles to adhere thereto. Upon standing and preferably with the application of a slight amount of heat the solvent is evaporated from the printing base.

What is claimed is:

l. A method of electrostatic printing comprising the steps of exposing a photoconductive insulating layer to an electromagnetic radiation image, said layer having an insulating coating on and integral with one surface thereof, the volume under the exposed areas of said layer combining in series with the volume of said coating under the same said areas to provide at least one electrical network having a predetermined time constant, equal to RL(Ci-|Cp) wherein: RL is the resistance of the volume under an exposed area of said layer, Ci is the capacitance of the volume of said coating under the same said area and Cp is the capacitance of the volume of said layer under the same said area, Ci being substantially larger than Cp, applying an electric field across said layer and said coating for a time about equal to RD being the resistance of the same volume of said layer when unexposed and having a value at least l0 times the value of RL, to provide an electrostatic image on said coating, and developing said electrostatic image by applying thereto a finely-divided developer substance.

2. The method of claim 1 wherein said electric eld is applied across said layer and said coating during said projecting step.

3. The method of claim 1 wherein said electric eld is applied across said layer and said coating subsequent to said projecting step.

4. The method of claim 1 including a substantially transparent conductive electrode in contact with the other surface of said photoconductive layer, said electromagnetic radiation image being projected onto said photoconductive layer through said electrode.

5. The method of claim 1 wherein said insulating coating is substantially transparent and said electromagnetic radiation image is projected onto said photoconductive layer through said coating.

(References on following page References Cited in the le of this patent UNITED STATES PATENTS 10 Owens May 12, 1959 Dessauer et al. Aug. 25, 1959 Schaiert et al Aug. 25, 1959 Moncrieff-Yeates Sept. 15, 1959 Schatert Nov. 29, 1960 Johnson et al Nov. 28, 1961 OTHER REFERENCES Wainer: Photographic Engineering, vol. 3, No. 1, pp. 

1. A METHOD OF ELECTROSTATIC PRINTING COMPRISING THE STEPS OF EXPOSING A PHOTOCONDUCTIVE INSULATING LAYER TO AN ELECTROMAGNETIC RADIATION IMAGE, SAID LAYER HAVING N INSULATING COATING ON AND INTEGRAL WITH ONE SURFACE THEREOF, THE VOLUME UNDER THE EXPOSED AREAS OF SAID LAYER COMBINING IN SERIES WITH THE VOLUME OF SAID COATING UNDER THE SAME SAID AREAS TO PROVIDE AT LEAST ONE ELECTRICAL NETWORK HAVING A PREDETERMINED TIME CONSTANT, EQUAL TO RL(C1+CP) WHEREIN: RL IS THE RESISTANCE OF THE VOLUME UNDER AN EXPOED AREA OF SAID LAYER, C1 IS THE CAPACITANCE OF THE VOLUME OF SAID COATING UNDER THE SAME SAID AREA AND CP IS THE CAPACITANCE OF THE VOLUME OF SAID LAYER UNDER THE SAME SAID AREA, C1 BEING SUBSTANTIALLY LARGER THAN CP, APPLYING AN ELECTRIC FIELD ACROSS SAID LAYER AND SAID COATING FOR A TIME ABOUT EQUAL TO ((RL)(RD)(CI+CP)LN(RD/RL))/RD-RL RD BEING THE RESISTANCE OF THE SAME VOLUME OF SAID LAYER WHEN UNEXPOSED AND HAVING A VALUE AT LEAST 10 TIMES THE VALUE OF RL, TO PROVIDE AN ELECTROSTATIC IMAGE ON SAID COATING, AND DEVELOPING SAID ELECTROSTATIC IMAGE BY APPLYING THERETO A FINELY-DIVIDED DEVELOPER SUBSTANCE. 